The PSP or Playstation Portable is one of the greatest handhelds, infact i still own 4 of them, heres the news:

Davee has posted an interesting article over at describing the 6.61 Infinity internals beginning with a brief history lesson about PSP firmware in general. He mentions 2.71 SE-A as the first custom firmware for PSP, but I remember Dark_Alex first proof of concept firmware based on 1.50 released with source code before that. If anyone is interested it can be found here. Otherwise the article covers the technical aspects really well, Davee goes into depth explaining the bugs used to exploit the system and the process of restoring system stability, providing a recovery system, then finally load the 6.61 firmware.